1. Technical Field
This invention relates to gas turbine airfoils in general, and to methods and apparatus for refurbishing gas turbine airfoils in particular.
2. Background Information
Gas turbine engines, particularly those in aircraft applications, will occasionally ingest substances (e.g., water and sand) entrained within air drawn into the engine. The substances will wear rotorblades and stator vanes ("airfoils") located within the engine. The leading edge of an airfoil is particularly susceptible to this type of damage. Left unchecked, deformation and erosion will negatively affect the performance of the airfoil and can eventually cause irreparable damage to an airfoil. To minimize performance loss and to ensure safe operation, airfoils are periodically inspected for deformation and wear both on-wing and during regularly scheduled overhaul and maintenance. In those instances where the wear is beyond acceptable standards, the rotor blade or stator vane must be refurbished or replaced. A person of skill in the art will recognize that airfoils within a gas turbine engine, particularly fan blades within modern high-bypass ratio fan blades, are very expensive to replace. Hence, there is considerable advantage in refurbishing gas turbine airfoils when possible.
If, for example, the leading edge of a fan blade is worn beyond acceptable standards, present refurbishment methods generally require that the aircraft be taken off-line, and the fan assembly subsequently removed from the engine and disassembled so the worn airfoil can be refurbished. If the wear is within predetermined limits, the leading edge is refurbished by machining the leading edge back to or near original specifications Although refurbishing a blade using presently known techniques is preferable to replacing the blade, there are nevertheless several undesirable aspects associated with such a process. First, the aircraft is typically taken out of service thereby eliminating its revenue producing potential. Second, there is considerable labor and cost involved in removing the blade from the engine particularly when the engine is mounted on-wing. In addition, when a rotor assembly is disassembled it is sometimes necessary to perform a "run-up" test before the engine can be allowed back in service. Testing of this nature, while prudent and necessary, nevertheless also increases the cost of the repair. Third, fan blade leading edges are typically refurbished using a manual process. The accuracy of the refurbishment is important because the leading edge profile is critical to the aerodynamic performance of the airfoil, and consequent performance of the engine. Accurately refurbishing the edge by hand requires considerable skill and time and is generally considered to be a long lead-time process. This is particularly true for ultra-high bypass fan blades that have significant twist and curve.
In instances where a foreign object more substantial than water or sand (e.g., rocks, birds, etc.) is ingested into the engine and impacts a rotor blade or stator vane, quite often a nick or dent occurs too large to be accommodated by refurbishment. If the damage is within allowable standards, a blending operation can be used to repair the damage. To our knowledge, blending operations do not restore the airfoil leading edge to its original profile. In fact some practices involve removing a curved portion out of the edge to eliminate the damage. The depth of the curved portion into the airfoil causes the repaired edge to be blunter than the original edge. A person of skill in the art will recognize that a blunter edge, even one that is only slightly different, will likely have appreciable impact on the aerodynamic performance of the airfoil. Another problem with some blending operations is that they involve forming tools that are not completely guided or have only limited guidance relative to the edge to be blended. Limited guidance cutting tools are often harder to control making it harder for the operator to produce the desired leading edge profile. In addition, if used improperly, a blending tool can gouge and irreparably damage an airfoil during the repair process. For these reasons, blending tools are not well suited for edge refurbishment.
What is needed, therefore, is a method and/or an apparatus for refurbishing gas turbine airfoils that can be used on airfoils mounted within a gas turbine engine, and one that does not require considerable skill to perform or use.